In FIG. 1a, there is shown a conventional index-coupled type, distributed feedback (DFB) semiconductor laser device that is shown, for example, in Japanese Patent Publication No. SHO 60-66484 laid open to public inspection on Apr. 16, 1985. The laser device of FIG. 1a comprises an N-type GaAs substrate 21, a first cladding layer 22 of N-type Al.sub.0.4 Ga.sub.0.6 As formed on the substrate 21, an intrinsic (I) Al.sub.0.1 Ga.sub.0.9 As active layer 23 formed on the first cladding layer 22, a P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24 formed on the active layer 23, and an N-type GaAs current blocking layer 25 formed on the beam guide layer 24. As will be described later with reference to FIG. 1b, corrugations 31 and 32 each comprising periodically repeating ridges are formed, by etching, in the upper surface portion of the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24, in a groove 30 in the N-type GaAs current blocking layer 25 and in the upper surface of the current blocking layer 25, respectively. A second cladding layer 26 of P-type Al.sub.0.4 Ga.sub.0.6 As is formed over the groove 30 to fill it and also over the upper surface of the N-type GaAs current blocking layer 25. Over the second cladding layer 26, is formed a P-type GaAs contact layer 27. An electrode layer 28 of, for example, Cr/Au overlies the P-type GaAs contact layer 27. An electrode layer 29 of, for example, Au-Ge/Au is formed to cover the bottom surface of the N-type GaAs substrate 21.
This semiconductor laser device is fabricated in a manner stated below. For example, an MOCVD (metalorganic chemical vapor deposition) technique is employed to successively grow, on the N-type GaAs substrate 21, the N-type Al.sub.0.4 Ga.sub.0.6 As first cladding layer 22, the intrinsic Al.sub.0.1 Ga.sub.0.9 As active layer 23, the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24 and the N-type GaAs current blocking layer 25. Next, referring to FIG. 1b, a photoresist layer is formed over the current blocking layer 25, and is exposed to a pattern of laser light, using a two-beam interference exposure technique, to thereby form a predetermined periodic striped pattern in the photoresist layer. Next, the current blocking layer 25 is etched by the reactive ion etching technique, with the stripe-patterned photoresist layer used as a mask, whereby the corrugation 32 comprising periodically repeating ridges is formed in the current blocking layer 25. After that, a photoresist mask is formed over the current blocking layer 25, and the groove 30 of a predetermined width is formed to extend in the direction perpendicular to the direction of extension of the ridges. The conditions for this etching are determined such that the etching advances vertically but etching from the sides of the ridges is prevented. Then, the periodic stripe pattern in the N-type GaAs current blocking layer 25 is retained during the etching and, ultimately, it is transferred to the surface of the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24.
Thereafter, the photoresist is removed and the surface of the structure is cleaned. The MOCVD technique is again used to successively grow the P-type Al.sub.0.4 Ga.sub.0.6 As second cladding layer 26 and the P-type GaAs contact layer 27. Then, the Cr/Au electrode layer 28 and the Au-Ge/Au electrode layer 29 are deposited over the contact layer 27 and the bottom surface of the substrate 21, respectively.
Next, operation of the above-stated index-coupled type semiconductor laser device is described. The electrode layers 28 and 29 are connected respectively to the positive and negative terminals of a bias source (not shown). Then, current flows through the semiconductor laser so that carriers injected into the intrinsic Al.sub.0.1 Ga.sub.0.9 As active layer 23 recombine, whereby light is emitted. As the injection current level is increased, stimulated emission begins, and, ultimately, laser oscillation results. Part of laser light is guided into the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24. The period .LAMBDA. of the ridges in the corrugation 31 in the surface of the beam guide layer 24 in the groove 30 is determined to be. EQU .LAMBDA.=m.multidot..lambda..sub.0 /2N.sub.r ( 1)
where:
m is a positive integer; PA1 N.sub.r is an index of refraction of the beam guide path; and PA1 .lambda..sub.0 is an oscillation wavelength.
Then, only light of the wavelength .lambda..sub.0 is selected and, accordingly, single longitudinal mode oscillation results.
During the manufacturing of the above-described conventional AlGaAs index-coupled type DFB semiconductor laser device, the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24 is exposed to air i.e., during the photoresist removal and surface cleaning treatments. Since the beam guide layer 24 contains Al, the surface of the beam guide layer 24 is oxidized. The P-type Al.sub.0.4 Ga.sub.0.6 As second cladding layer 26 is then formed through crystal growth over the corrugation 31 of the P-type Al.sub.0.25 Ga.sub.0.75 As beam guide layer 24, the surface of which has been oxidized. This results in a number of crystal defects in the vicinity of the hetero-interface between them, which degrades the crystallinity of the layers. Such crystal defects tend to increase during the operation of the semiconductor laser device, which shortens the life of the device.
There is another type of DFB semiconductor laser device, namely, a gain-coupled type DFB semiconductor laser device. An example of the gain-coupled type DFB semiconductor laser device is shown in FIGS. 2a and 2b. This laser device comprises an N-type GaAs substrate 41, an N-type Al.sub.0.40 Ga.sub.0.60 As first cladding layer 42 formed over the substrate 41, a P-type GaAs active layer 43 overlying the first cladding layer 42, a P-type Al.sub.0.25 Ga.sub.0.75 As carrier confining layer 44 overlying the active layer 43, and a P-type GaAs beam guide layer 45 formed to overlie the carrier confining layer 44. As shown in FIG. 2b, corrugation 51 comprising periodically repeating ridges repeating with a period .LAMBDA. is formed in the surface of the P-type GaAs beam guide layer 45. Overlying the corrugated P-type GaAs beam guide layer 45, a P-type Al.sub.0.40 Ga.sub.0.60 As second cladding layer 46 is formed, which in turn is overlain by a P-type GaAs contact layer 47. This device also includes electrode layers 48 and 49 similar to the ones used in the semiconductor laser device shown in FIG. 1a.
The gain-coupled type DFB semiconductor laser device shown in FIGS. 2a and 2b oscillates in the single longitudinal mode, when the electrode layers 48 and 49 are connected to positive and negative terminals, respectively, of a bias voltage source, which causes current to flow through the semiconductor laser device. By selecting the value in accordance with the aforementioned equation (1) for the period .LAMBDA. of the corrugation 51, only light of the wavelength .lambda..sub.0 is selected, and the device oscillates in the single longitudinal mode at the wavelength .lambda..sub.0.
When the gain-coupled type DFB semiconductor laser device shown in FIGS. 2a and 2b is fabricated, the corrugation 51 is formed in the P-type GaAs beam guide layer 45, and, therefore, the surface exposed to air during the manufacturing is the upper surface of the P-type GaAs beam guide layer 45 which does not contain Al. Since spontaneous oxidation of the surface of the layer 45 is limited, the number of crystal defects at the re-grown hetero-interface as compared to the aforementioned index-coupled type DFB semiconductor laser device is less. Accordingly, this gain-coupled type DFB semiconductor laser device has no degradation of reliability or shortening of life caused by the degradation of the crystallinity. However, this device has other problems. That is, since the P-type GaAs beam guide layer 45 itself absorbs laser light, the internal loss within the laser resonator is large so that the laser device tends to have an increased laser oscillation threshold current, a decreased quantum efficiency.
An object of the present invention is to provide an AlGaAs semiconductor laser device, in particular, an AlGaAs gain-coupled type DFB semiconductor device which is free of the above-stated various problems and has a low oscillation threshold current, high quantum efficiency, high reliability and a long life. The present invention also provides a method of fabricating such a semiconductor laser device.